A. Field of Invention
The present invention relates, in general, to an apparatus, an electrolyte, and a process for electrochemically microfinishing metals provided in the form of a strip or a band. More particularly, the present invention relates to an apparatus and to an electrochemical process incorporating an electrolyte solution including glycerol for electropolishing and electroetching stainless steel print bands.
B. Description of Related Art
In the manufacture of print bands used in high-speed, impact printers, final surface finishing is an important step. The surface of these print bands, which are typically hardened stainless steel, must have special characteristics in order to resolve properly the tradeoff between ribbon life and print quality. Currently, mechanical buffing is used for final finishing of print bands.
Buffing is done by nylon brushes impregnated with an abrasive such as alumina, TiC, or the like at the brush tips. The tips of the brushes break off unevenly during the finishing process, rendering the process highly irreproducible. The buffing process is also relatively slow and yields an inferior surface finish. Specifically, although buffing removes the original surface roughness to a certain extent, it introduces numerous scratches which are unevenly distributed over the surface. Moreover, mechanically induced stresses are present in the surface following buffing.
Certain printer applications demand that the characters on the print band be rounded with a different degree of rounding at the leading and trailing edges. Such rounding requires a degree of carefully controlled metal removal during final finishing.
To obtain the desired microfinish of, and the desired degree of metal removal from, the stainless steel print bands, electropolishing and electroetching are herein suggested. The technological aspects of electrolytic polishing of stainless steels, including operating conditions and electrolyte composition, are well documented. See, e.g., J. F. Jumer, Metal Finishing Guidebook Directory at 513 (Metals and Plastics Publications, Hackensack, N.J., 1972); W. Schwartz, 68 Plating and Surface Finishing at 42 (June 1981); I. Rajagopalan, Finishing Industries at 27 (Sept. 1978); S. J. Grilichies, Electrochimicheskoje polirowanie (Leningrad, 1976); P. V. Shigolev, Electrolytic and Chemical Polishing of Metals (Freund, Tel-Aviv, 1974); W. J. McTegart, The Electrolytic and Chemical Polishing of Metals (Pergamon Press, London, 1956); J. P. Hoare & M. A. LaBoda, 2 Comprehensive Treatise of Electrochemistry (J.O'M Bockris, B. Conway, E. Yeager & R. E. White eds., Plenum Press, 1981); L. Ponto, M. Datta & D. Landolt, 30 Surface and Coatings Technology at 265 (1987). These references indicate that electrolytic polishing of stainless steels on an industrial scale is most easily done in concentrated phosphoric acid-sulfuric acid solutions.
Electrolytes based on perchloric acid-acetic acid have also been used on a laboratory scale. W. J. McTegart, Electrolytic & Chemical Polishing of Metals (Pergamon Press, London, 1956). Perchloric acid with organics such as acetic acid or acetic anhydride are seldom used today, however, because they have an explosive nature. Accordingly, solutions based on a mixture of phosphoric and sulfuric acids are more important. W. J. McTegart, Polissage electrolytique et chimique des Metals (Dunod, Paris, 1960).
For electropolishing of stainless steels, the known solutions sometimes have additives and the electrolytic process is conducted at elevated temperatures. Several patents and publications have mentioned the use of different additives, including glycerol. See, for example, U.S. Pat. No. 2,315,695 (Faust); P. V. Shigolev, Electrolytic & Chemical Polishing of Metals, (Freund, Tel-Aviv, 1974); W. J. McTegart, Electrolytic & Chemical Polishing of Metals (Pergamon Press, London, 1956). None of the electropolishing baths disclosed, however, take into consideration the manufacturing aspects; therefore, they are not directly applicable for microfinishing in the print band manufacturing process.
For example, the 59-64% glycerol bath mentioned in the '695 patent would involve a high cell voltage, creating electrolyte heating and a large power requirement. For electropolishing materials with spring-type characteristics, excessive heating would destroy such properties. Most of the glycerol-containing baths mentioned in the literature operate at high temperatures (40-90 degrees Centigrade). Moreover, such baths generally contain further additives, thus making them difficult to adapt to manufacturing processes in the electronics industry.
The surface finish obtained by using the known processes is generally very sensitive to changes in operating conditions--in particular, current density, temperature, and hydrodynamic conditions. Specifically, with respect to current density, prolonged application of relatively high currents (up to 60 amperes) to thin, moving print bands (about 150 microns thick) may create heating problems. Especially at the point of electrical connection, such problems may include sparking and burning of the band. High current requirements also demand high cell voltage and, consequently, high power supplies. Unfortunately, such problems are difficult to avoid if sufficient anodic dissolution cannot be obtained without using high current densities.
Depending upon the electrolyte solution and the operating conditions used, anodic dissolution of a metallic workpiece may lead to any one of the following: (1) anodic etching, revealing crystallographic steps and etch pits, preferred grain boundary attack, or finely dispersed microtexture; (2) partial or complete passivation; and (3) electropolished surfaces. Oxygen evolution may accompany the metal dissolution reaction which occurs during electropolishing. Moreover, the success of electropolishing depends upon the prevailing mass transport and current distribution conditions and on the ability to form surface films on the dissolving anode. These factors, in turn, depend upon the specific metal-electrolyte interaction, hydrodynamic conditions, applied current density and cell voltage, and the cell geometry. Development and control of an electrolytic process, therefore, requires control of the interaction between these parameters and the influence of the parameters on the resulting surface finish of the metal.
In addition to the electrochemical factors which govern the process of electrolytic finishing, optimization and successful application of the process depend on several other factors. Such factors include: metal composition, grain size, inclusions, initial surface state, and initial surface roughness. Although an electrolytic process can create highly reflecting, mirror-like, microscopically flat surfaces, such results are obtained generally only for pure metals and homogeneous alloys containing small amounts of inclusions. Successful electropolishing of two-phase alloys, on the other hand, is much more difficult to achieve.
That difficulty is caused, in part, by differences in the rates of dissolution of the different phases, creating extremely rough surfaces. For similar reasons, anodic dissolution of alloys containing significant amounts of inclusions yields pitting and other forms of localized attack. Thus, to develop a successful electrolytic process for microfinishing such materials, conditions which suppress localized and preferential dissolution must be ascertained.
The significant amount of inclusions present in the stainless steels used to manufacture print bands renders electropolishing of such materials difficult. Previously known electrolytic solutions and conditions, and the devices used to apply those solutions and conditions, have proven inadequate.
With the above discussion in mind, it is one object of the present invention to provide a completely automated device able to electropolish and electroetch materials provided in strip form, such as print heads. A related object is to provide a device which includes provisions for selective removal of material from the corners of the characters, giving enhanced control over the character profile, and for uniform levelling and microfinishing of the entire print band surface. A further object is to provide an apparatus which reduces the number of passes of the print band required to obtain acceptable surface finish, thereby increasing output. Another object is to provide an apparatus able to produce better reproducibility and better surface finish than existing devices. Finally, another object is to reduce the current required by the apparatus.
The following objects are attendant the electrolytic process of the present invention. The process should: (a) be non-explosive and should not contain toxic components; (b) be operable at ambient temperature and be insensitive to small variations in electrolyte temperature, thereby minimizing losses due to evaporation generally encountered in processes which operate at elevated temperatures; (c) involve minimal agitation of the electrolyte, thus eliminating the high cost involved in pumping concentrated acids; (d) provide microfinishing at relatively low current density; (e) provide a desired and controlled material removal rate; and (f) ensure safety.
Another object is to provide an electrolyte with a sufficiently high conductivity so that power requirements are relatively low. A related object is to assure that the electrolyte is relatively noncorrosive and remains stable, without polymerization or other degradation, over long periods of time.